This patent application is co-pending with a related patent application Ser. No. 09/443,241 by the same inventor as this patent application.
(1) Field of the Invention
The present invention relates generally to communication systems, and more particularly to a method for using an adaptive modem that selects modulation and signal processing schemes to minimize detection, demodulation, and decoding errors while maximizing transmission data rates.
(2) Description of the Prior Art
Doppler and multipath are two well-known effects presented by communication channels. The doppler effect is characterized by an alteration of the transmitted signal frequency when there is relative movement between the transmitter and receiver. Alternately, multipath describes the multiple and time-delayed received signals resulting from a single transmission that travels multiple and varying length paths between the transmitter and receiver. U.S. Pat. No. 5,301,167 to Proakis et al. describes the ocean acoustic channel behavior and the resulting variant multipath that must be recognized as a basic channel characteristic. Proakis et al. describe an underwater acoustic communications apparatus using doppler removal, a specialized sample timing control technique, and decision feedback equalization to achieve high data rates for phase coherent modulation and demodulation. Proakis et al. present an apparatus in which digitized data streams are preceded by a synchronization signal and training sequence that are Phase-Shift-Keyed (PSK) modulated. This technique, although more efficient than other Frequency-Shift-Keying (FSK) techniques, provides a single transmit modulation and receive demodulation scheme regardless of channel characteristics. Experimentation with the FSK technique indicates communication difficulties even with a relatively high signal-to-noise ratio (SNR).
There is currently no method that measures the channel""s time and frequency characteristics to adapt the transmission modulation scheme.
What is needed is a method that actively measures the communication channel multipath and doppler characteristics, and uses such multipath and doppler measurements to determine communication system modulation and demodulation schemes.
It is a general purpose and object of the present invention to improve communication system error and transmission data rates. It is another object of the present invention to provide such improvement through a method that utilizes adaptive modems to measure the communication channel""s doppler and multipath characteristics. A further object is to estimate the communication channel""s scattering function using the doppler and multipath measurements and modem position estimates. Still another object of the present invention is to utilize such communication channel scattering function, together with channel characteristic data, strategic information, and propagation models, to select one of multiple modulation schemes for the communication scenario.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
These objects are accomplished with the present invention by a method that utilizes an adaptive modem to estimate the communication channel scattering function and select one of multiple modulation schemes. The scattering function is derived from measurements of the channel""s frequency (doppler) and time (multipath) spreading characteristics. The communication channel""s doppler and multipath are measured by a channel probe signal that is transmitted from a first modem to a second modem. The second modem processes the received channel probe signal and transmits the channel spreading factors to the first modem. The first modem combines the channel spreading factors and modem position estimates to estimate the channel scattering function. The first modem uses the channel scattering function estimate, channel characteristic data, strategic information, and propagation models to select the optimum available modulation scheme, each modulation scheme comprised of a modulation technique and encoder. Subsequent data transmissions from the first modem to the second modem contain data modulated by the selected modulation scheme and coded with a modulation mode identifier. The second modem uses the modulation mode identifier to select a demodulation scheme comprising a demodulation and decoder pairing. Upon the occurrence of predetermined criteria, the channel scattering function estimate may be updated and a new modulation scheme may be selected to continue transmission. Because each modem is equipped with the probe signal, bi-directional channel characterization can be performed.